Cancer arises during the process of carcinogenesis, in which the exposure of cells to genotoxic insults leads to the introduction of permanent genetic changes. The ability of cells to detect this DNA damage and to activate appropriate repair mechanisms is crucial for the cell to suppress carcinogenic events. The Tip60 histone acetyltransferase is a key mediator of the cells ability to detect and repair these DNA lesions. Tip60 is a key component of the NuA4-Tip60 chromatin remodeling complex. Chromatin remodeling is utilized to unpack higher ordered chromatin structures and alter histone-DNA interactions to facilitate access of the DNA repair machinery to DNA lesions. Our results demonstrate that chromatin remodeling by the NuA4-Tip60 complex requires Tip60's HAT activity. Further, Tip60 contains a chromodomain, a conserved domain with the potential to interact with methylated lysine residues on histones, and this domain is required for the activation of Tip60's HAT activity and NuA4-dependent chromatin remodeling. The long term aims are to test the hypothesis that Tip60 is a key component of a novel signal transduction pathway which links the detection of DNA damage to alterations in chromatin structure. We will test the hypothesis that the catalytic components of NuA4, including the p400 ATPase activity and Tip60's HAT activity, are required for chromatin remodeling following DNA damage. Further, we propose that interactions between the chromodomain of Tip60 and exposed methyl-lysine residues on histones at sites of DNA damage mediate this activation of Tip60's HAT activity. Tip60 exists as a trimeric complex containing the Tip60, epc1 and ING3 proteins. In specific aim 1, the role of the ING3 regulatory protein in controlling Tip60's HAT activity will be defined, and the mechanism by which NuA4 is recruited to DNA breaks will be determined. In specific aim 2, the function of the ATPase activity of the p400 sub- unit of NuA4 in chromatin unwinding will be determined, and the contribution of histone acetylation by Tip60 at sites of DNA damage identified. In specific aim 3, the specificity of interaction between the chromodomain of Tip60 and methylated lysine residues on histones will be determined, and the role of histone methylation in the DNA-damage dependent activation of Tip60 examined. An understanding of the signal transduction pathway by which Tip60 detects DNA strand breaks caused by bleomycin and related agents will provide crucial insights into the cytotoxicity and mutagenic properties of these agents. A more complete understanding of how Tip60 regulates the DNA damage response may allow new therapies, including the rational design of novel therapeutic compounds, to be applied to cancer therapy and to clinically beneficial interventions for reducing the risk from genotoxic stress. Finally, Tip60 and its associated proteins are potential targets for developing therapeutic agents which can modify the DNA damage response of tumor cells.